In construction, modular panel systems are commonly used to reduce build cost and construction time. Modular panel systems typically allow for the rapid construction of floors, walls and ceilings, albeit often at the compromise of reduced aesthetic appearance. A prime example of such a modular panel system is a suspended ceiling, which can be found in most professional environments, such as for example office spaces. A suspended ceiling typically comprises a metal or plastic grid defining rectangular or square recesses, which are filled with tiles to form a continuous ceiling.
In such modular systems, e.g. a suspended ceiling, lighting may be integrated into the system, typically by replacing one or more tiles with a lighting unit such as a luminaire. Most suspended ceilings comprise luminaires in which a number of fluorescent light tubes are present. For a number of reasons, such luminaires are not ideal. Firstly, such luminaires are considered aesthetically displeasing, i.e. obtrusive. Secondly, in order to improve light efficiency from such luminaires, they usually contain a reflector, which commonly has a parabolic shape. This however can cause glare for an occupant of the office space if the reflected light exits the luminaire under shallow angles to the plane of the modular system. Glare can be very disturbing in an office environment, as it can obscure the image on a computer monitor and can cause physical discomfort, e.g. headaches or sight problems, to the occupant when being subjected to the glare for a prolonged period of time. This is why health and safety standards such as the IEC60598-1:2008 standard in Europe require lighting solutions to comply with stringent requirements for preventing excessive glare levels.
Solutions exist to overcome glare. One solution involves luminaires comprising a plurality of chambers defined by respective parabolic reflectors, with a fluorescent light tube fitted in each of the chambers. Each of the light tubes is offset from the light exit plane of the luminaire such that light emitted by the fluorescent light tube under a shallow angle is reflected by the parabolic reflector, which increases the exit angle of the light, thus reducing glare. A drawback is that this solution leads to relatively bulky luminaires, which can be considered aesthetically unsatisfactory.
Another solution involves fitting luminaires for integration in a suspended ceiling with a micro-lens optical (MLO) plate or diffuser, which has the function of preventing shallow angle light beams from exiting the luminaire. As this causes a substantial amount of light being reflected back into the chamber of the luminaire, the luminaire may comprise a reflector to recycle such reflected light. Commonly, the micro-lens optical plate takes the form of a prism plate. Again, this style of luminaire is relatively bulky and obtrusive.
The company SwitchMade offer a light emitting diode based (LED) luminaire marketed under the name Paneos® for integration in a suspended ceiling. This has the advantage of lower energy consumption compared to fluorescent light tube-based luminaires.
However, as these luminaires replace tiles in the ceiling, they still disrupt the visual appearance of the suspended ceiling.
One important option for unobtrusively integrating lighting fixtures into the ceiling system is shown in FIG. 1. In this system thin lighting fixtures 1 are fixed onto support strips 2. The width of such a fixture 1 would typically be 25 mm with a thickness of a few millimeters. A set of such fixtures 1 would typically span the whole width or length of a room, yielding continuous “lightlines”. A typical office ceiling would be equipped with multiple “lightlines” spaced approximately 60 cm apart. Ceiling tiles 3 can be suspended on the lighting fixtures 1 as shown.
The Gemino Company (www.gemino.it) markets a suspended ceiling solution in which the light fittings can be integrated into the band raster of the ceiling. The band raster consists of the main structural beams of the suspended ceiling. With smaller form factor lighting such as LED lighting this is a feasible solution, and has the advantage of the improved appearance of the ceiling due to the fact that no tiles need replacing with luminaires.
It is important with this kind of lighting fixture that the brightness is more or less uniform over the emission surface. If the surface contains high brightness non-uniformities, this can lead to glare and discomfort in the case of direct viewing. Since strip mounted lighting fixtures have a relatively small emitting area, the glare requirements are quite strict and an evenly distributed emission over the fixture surface is needed. Besides this, an even distribution is aesthetically more pleasing.
One successful design of strip mounted lighting fixture is shown in FIG. 2. It consists of a transparent light guide 4 (made from PMMA, for example), coated with a diffuser 5 and, optionally, with a reflector 6. The thickness of the light guide 4 is typically 1-2 mm. Light is generated by LEDs 7 at the sides and is coupled into the light guide 4. If the diffuser 5 were absent, the light would be guided by total internal reflection (TIR) towards the opposite side of the guide, and exit. The diffuser 5 serves to scatter the light out of the light guide 4. Once outside the light guide 4, the light encounters a micro lens optics (MLO) plate 8. This MLO plate 8 contains pyramid-like structures. It has the property that only light that exits the plate within a half-cone-angle of approximately 60° is transmitted. Light that is not transmitted is recycled; it returns towards the light guide 4 where it is redirected by the diffuser 5, and gets a second chance to escape.
If the LEDs are closely packed along the edges of the light guide 4, this design shows a pleasing, homogeneous light output over the surface of the MLO plate 8. In practice however, a high density closely packed LED distribution will generate too much light for the applications envisioned. With today's low power packages, the LEDs have to be spaced 1 to 2 cm apart. It is envisioned that lumen output of LEDs will increase significantly in the near future, and the spacing between LEDs will increase accordingly. Besides this, cost issues will probably stimulate the use of medium power or even high power LEDs. It has been found that when spacing the LEDs further apart, the homogeneity rapidly diminishes and becomes unacceptable.
This issue is exacerbated if the fixture is made color-tunable by using a mix of different color LEDs, like for instance warm white and cold white. In that case, the spacing per color increases even further. It has been found that using warm white and cold white LEDs at a LED-to-LED distance of 1 cm results in a surface light distribution that is very inhomogeneous. The positions of LED sources become clearly visible, having relatively high brightness, and mixing between the colors is very poor.